1. Field of the Invention
The present invention relates to imaging systems, and more particularly to a method of forming images of objects using ultrasonic sound waves.
2. Description of Prior Art
Conventional echo ranging image systems such as those using ultrasonic sound waves employ signal pulses with spectra of relatively narrow bandwidth centered at a carrier frequency. The received echoes are normally processed by a beamformer, such as a lens, phased array, holographic receiver, etc., to determine the direction of arrival, and by an envelope detector to determine pulse echo arrival time, and hence the distance to the object. The distance resolution is normally not sufficient to distinguish range differences to individual portions of the object. This last characteristic of conventional systems is due to the relationship between signal spectrum bandwidth and time duration of the pulse.
In order to secure sufficient pulse energy to overcome the attenuation of the medium, such as water, and with a physical limit on the peak power available for transmission set by transducers and electronics, pulses are required to be of a length such that they entirely encompass objects of interest. Therefore, the "depth of field" is great enough that portions of the object with high reflectivity, so-called specular highlights, completely dominate the system. Non-linearities of receiver response and leakage between angular resolution cells often cause complete saturation of the receiver. Large portions of the object are thus "overexposed". What is even more serious is that visual clues for recognition of the shape of the object most often originate from surfaces where reflectivity is low. The display, therefore, contains large amounts of high intensity reflected energy from specular facets of the object which "wash out" the desired low intensity reflections from which the nature of the object can be identified. Additionally, large regions of the medium surrounding the object may contain many small scatterers which cause the object to be obscured.